a) Field of the Invention
The present invention relates to a compact zoom lens system which has favorable optical performance.
b) Description of the Prior Art
In the recent years, zoom lens systems have vari-focal ratios of 2 and higher, and made compact optically by strengthening refractive powers of the lens elements used therein and mechanically by adopting collapsing mechanisms utilizing dead spaces remaining in the lens systems. These designs make it possible to satisfy the basic demands for imaging systems, concretely portability, operability and so on. Since zoom lens systems having long total lengths and large outside diameters inevitably enlarge lens barrels to be used therewith, it is desired to design compacter zoom lens systems so far as they are not to be used in special fields of application and the compacter design of a lens system constitutes an important requirement especially in a case where an optical system is to be incorporated in a photographing system.
The inventor proposed, by Japanese Patent Kokai Publication No. Sho 62-209508 (U.S. Pat. No. 4,789,229), a zoom lens system which has a high zooming ratio. One of the object of the invention disclosed by U.S. Pat. No. 4,789,229 was to compose a zoom lens system of lens elements which are not larger than required without degrading optical performance thereof. In other words, there lies a certain limit in the attempt to make a lens system compacter by strengthening refractive powers of the lens units to compose said lens system since the strengthening the refractive powers of the lens units makes it necessary to impose stricter manufacturing allowance on the lens elements which are to compose the lens elements, whereby the lens elements can hardly be manufactured in practice. For this reason, another object of the above-mentioned patent was to make the zoom lens system lens susceptible to manufacturing errors of the lens elements.
It is conventionally considered that compact design of an optical system is conflicting with enhancement of optical performance thereof, and one of the problems on lens design is to properly balance compact design with optical performance.
In selecting a refractive power distribution which is determined by refractive powers of lens units, i.e., partial lens systems, and distance between principal points, refractive powers of the lens units are to be strengthened for designing a lens system compact, but correction of aberrations becomes difficult when the lens units are composed of thick lens elements. Further, when the distance between the principal points of each of the lens units is shortened for designing the lens system compact, it is obliged to select a number of lens elements which imposes no restriction on arrangement of thick lens elements in each of the lens units. When each of the lens units is composed of a small number of lens elements, correction of aberrations becomes difficult and the lens elements must be manufactured with higher precision. In order to correct aberrations with a small number of lens elements, it is obliged to adopt aspherical surfaces and graded refractive index lens elements. However, aspherical surfaces and graded refractive index lens elements are to be used for improving optical performance, and cannot modify the principle of lens designs. Accordingly, the use of aspherical surfaces and graded refractive index lens element inevirably makes it necessary to specify stricter manufacturing precision or requires more sophisticated manufacturing techniques.